Geobiology最新文献

Present-day angiosperm plants produce a plethora of metabolites including pigments that serve for important functions such as photosynthesis, protection against light, attraction of pollinators, and defense against microbes and herbivores. However, little is known about phytochemical constituents of ancient angiosperms, their distribution in the fossil record, their stability in deep time, and diagenesis. Outstanding preservation of ancient angiosperms, including exceptional color preservation, has been reported, but chemical analyses of such valuable specimens are limited by the rarity of the fossil material and the small amounts of potentially preserved metabolites. Here we use highly sensitive targeted liquid chromatography-tandem mass spectrometry in multiple reaction monitoring mode to screen for nanogram quantities of intact ancient phytochemical metabolites and their products in exceptionally well-preserved, about 45-Ma-old leaves from the Eocene Geiseltal fossil Lagerstätte, Germany. We show that diverse chlorophyll derivatives and degradation products as well as polyphenolic pigments are preserved in green to yellow colored angiosperm leaves and the brown coal matrix from Geiseltal. Most interesting is the fossil occurrence of the "unstable" green chlorophyll derivative dihydro-13²,17³-cyclopheophorbide a-enol, since cyclopheophorbide-enols are otherwise known as unique non-fluorescent chlorophyll catabolites of microorganisms in modern aquatic environments. The monopyrrole hematinic acid is interpreted as a stable product of chlorophyll catabolism via linear tetrapyrroles. Moreover, polyphenolic compounds in the fossil angiosperms are represented by the flavonoid pigments apigenin and luteolin. Our results demonstrate the potential of paleometabolomic-like screening of individual plant fossils to trace the fate of phytochemical constituents and to understand the processes of fossilization at the molecular level.

The Ediacaran–Cambrian boundary, which precedes one of the most significant biotic diversification events in Earth's history, is associated with a global negative carbon isotope excursion termed the BAsal Cambrian carbon isotope Excursion (BACE). Late Ediacaran and early Cambrian changes in shallow marine oxygenation have been proposed to relate to the BACE as well as metazoan extinction and radiation. However, reconstructing paleoredox conditions at the Ediacaran–Cambrian boundary is limited by challenges in correlating carbonate strata due to sparse stratigraphic markers and non-unique chemostratigraphic correlations. These imprecise correlations have led to uncertainty in how redox changes across the BACE should be interpreted in relation to broader regional and global environmental patterns. Here, we present redox reconstructions from southwestern Laurentian carbonate successions that record the BACE, including the limestone-dominated Deep Spring Formation, southwestern USA, and the dolostone-dominated La Ciénega Formation, northern Mexico. We combine local (carbonate-bound iodine, I/(Ca + Mg) and cerium anomaly, Ce/Ce*) and global (carbonate-associated uranium isotopes, δ²³⁸U_carb) redox proxies to investigate marine oxygenation in relation to the BACE. Contrary to previous suggestions that a global ocean oxygenation event coincided with the BACE, we do not observe a shift in δ²³⁸U_carb concurrent with the carbon isotope excursion in either section. The δ²³⁸U_carb values differ between two sections, likely reflecting distinct diagenetic offsets attributed to different diagenetic U reduction, but together provide a minimal constraint on the carbonate δ²³⁸U value and suggest a more anoxic ocean compared to today. The local proxy results at both sites suggest widespread low-oxygen surface waters with a transient and localized interval of shallow marine oxygenation at one site that coincides with the nadir of the BACE. Persistently low I/(Ca + Mg) ratios, below values observed in today's oxygenated oceans, suggest a broadly redox-stratified surface ocean. Negative Ce anomalies in the La Ciénega Formation were recorded during the BACE nadir, suggesting a short-lived interval of local oxygenation within otherwise low-oxygen conditions. In sum, we do not find evidence for major, widespread oxygenation coincident with the BACE, but a continuation of low-oxygen conditions punctuated by a short-lived oxygenation event in the shallow oceans. These brief fluctuations in oxygen levels, in turn, may have played a role in the onset of behavioral complexity among bilaterian invertebrates during this critical transition.

The Paleoproterozoic Earth underwent profound environmental changes, including multiple severe glaciations and fluctuations in atmospheric oxygen (O₂) levels. However, the precise relationship between O₂ evolution and the glaciations remains unclear. Here, we use a biogeochemical cycle model involving carbon, phosphorus, sulfur, and oxygen to investigate the redox dynamics of the ocean—atmosphere system following the climatic transition to a super-greenhouse state after deglaciation. Our stochastic analysis reveals that climatic recovery on a timescale of ~10⁵ years from elevated atmospheric CO₂ levels (> 0.2 atm) triggers an extensive oxidation of the atmosphere and oceans over the subsequent 10⁶–10⁷ years, aligning with the large sulfur isotope anomaly in buried pyrite after the third Paleoproterozoic glaciation (~2.3 Ga). This finding suggests that the third glaciation represented an extensively glaciated, snowball state, which would have required massive accumulation of atmospheric CO₂ for deglaciation. Variation in the boundary conditions regarding the global redox budget, as represented by high reductant fluxes, may explain the return of atmospheric O₂ to Archean-like levels following the first (Makganyene) glaciation, which is also considered a snowball Earth.

Ferruginous (iron-rich) conditions have been prominent in oceans throughout the Earth's geologic history but now are reliably found only in a handful of permanently stratified lakes. Microbially mediated iron reduction in such anoxic environments competes with sulfate reduction, which promotes euxinic (sulfide-rich) conditions. Besides the shared demand for organic compounds, the competition is fostered by the produced hydrogen sulfide, which may reduce iron oxides abiotically or co-precipitate with dissolved iron as iron sulfides. Understanding why some environments develop ferruginous rather than euxinic conditions (or vice versa), as well as the attendant effects on methanogenic fermentation, is key to understanding both modern and ancient anoxic ecosystems. Here, we reproduce biogeochemical distributions in multiple anoxic, low-sulfate, meromictic lakes around the world using a biomass-explicit reaction-transport model with a fixed set of metabolism-specific microbial parameters. The results suggest that sulfate reduction and methanogenesis are ubiquitous even in iron-rich systems, and are reflected in microbial surveys. Ferruginous conditions typically develop for surface sulfate concentrations below ≃100 μM. Interestingly, there seems to be a dearth of stably stratified water bodies where sulfate concentrations can persist in the medium-sulfate range of several hundred μM. Rather, when sulfur burial into the sediments becomes iron limited, sulfate tends to accumulate in the water column to much higher (mM) concentrations. A similar mechanism could be suggested to have operated in the variably sulfidic and ferruginous water columns of early oceans. Model simulations also reveal the previously underappreciated role of physical transport in shaping biogeochemical distributions, as minor variations in mixing rates can lead to large variations in microbial abundances. Model applicability across multiple lakes points to an encouraging possibility that geochemical patterns in complex biogeochemical systems may be described from a small number of thermodynamic and kinetic principles using a minimum of fitting parameters.

Tasik Biru is a ~70 m-deep tropical lake in Malaysia, originating from a water-filled open pit mine. We investigated the biogeochemistry and microbial community of the lake as a modern model habitat to the stratified ancient ocean. We found that a sharp redoxcline exists at around 50 m depth, related to the decrease of O₂ and pH (7.2–6.8) going down into the monimolimnion. Despite being relatively sulfate-rich (~320 μM), only a slight decrease of sulfate (to ~240 μM) was observed coupled with an increase of dissolved sulfide to 4 μM, attributed to microbial sulfate reduction in the monimolimnion. Comparatively, dissolved Fe and total Mn rose to ~50 μM in the anoxic layer with an unusual 1:1 concentration ratio. Other nutrients (PO₄³⁻, Si) and trace metal(loid)s (As, Mo, Sb, Co, U, and V) depth profiles increased or decreased across the chemocline, indicating controls via cycling of redox-sensitive elements. Microbial community analysis based on 16S rRNA amplicon sequencing reflects various metabolisms, from aerobic metabolisms in the mixolimnion to putative nitrite-dependent methane oxidation (e.g., by Methylomirabilis) at the chemocline, to sulfate reduction, methanogenesis, and fermentation in the monimolimnion. Tasik Biru is not in steady-state, and its anoxic water is predicted to shift from being Fe/Mn-rich to sulfide-rich, perhaps lending it as a model habitat to investigate biogeochemical changes from the metal-rich Archean to the Proterozoic oceans with expanding zones of sulfide-rich margins. An overview of the current biogeochemical cycles in the lake is presented, and open questions regarding partial sulfate consumption, methane, and Mn cycling and mineralogical distribution are highlighted to guide future studies.